The thickness of the semiconductor element and semiconductor wafer in which the element is incorporated plays a crucial part for applications such as smart cards, insulated gate bipolar transistors (IGBTs), high-power transistors and also diodes. In the case of the new generation of discrete power semiconductor components, it is thus possible to reduce the on resistivity and to minimize the forward voltage in the case of the diodes. Reducing the substrate thickness is advantageous in both of these cases. The invention relates to the processing of specially thin semiconductor substrates that contain the semiconductor elements.
The conventional steps developed in semiconductor technology are used in order to thin a wafer. The thick silicon wafers are generally ground thin only at the end of the process and processed differently to a conclusion. In this case, the processing may comprise steps such as ion doping, furnace processes, metallization or further steps. However, the processing of such thin substrates is a very great challenge since the silicon chips are very sensitive and can be damaged very easily. The minimum thickness of a wafer is therefore limited by the loss rate during the subsequent handling steps. Therefore, it is necessary to make a compromise in the thickness of the wafer and the yield. In order to minimize the loss rates, in accordance with the prior art, the wafers are additionally reinforced with a bonded-on plastic film in order to increase the mechanical stability. The difficulty in this method is that thin ground wafers are extremely fracture-sensitive, so that the wafer may be damaged when resolving the plastic film. Therefore, use is made of adhesives which lose their adhesive strength almost completely after UV exposure. However, the adhesives are not thermostable, so that wafers can only be processed under more difficult conditions.
The prior art also discloses further methods such as applying the wafer to a holding device, to which the wafer is applied and remains during the thinning of the wafer for some or all further process steps. These holding devices, described in EP 1 217 655, enable the wafer to adhere electrostatically to the holding device, and a certain mechanical stability of the wafer is thereby ensured. It has been shown, however, that the mechanical stability in the case of such a device does not suffice to minimize the loss rate since there is still a certain risk of fracture after the wafer has been stripped away from the holding device. According to the current prior art, the thickness of 75 μm for a 6-inch wafer (150 mm) is the absolute limit for handling and further processing. For an 8-inch wafer (200 mm), the limit is 175 μm.